The present invention relates to a two-unit zoom lens system and, more particularly, to a zoom lens system with a short back focus which is suitable for use with a compact camera or the like and which is composed of five or six lens elements and has a zoom ratio of the order of 2 to 2.5.
With the reduction in the cost of compact cameras, zoom lens systems having a smaller number of constituent lens elements have been proposed in recent years. Prior art whereby the number of constituent lens elements is reduced to four includes Japanese Patent Application Laid-Open (KOKAI) Nos. 3-127008 (1991) and 3-274516 (1991). In the prior zoom lens systems, the 1-st lens unit is composed of a pair of negative and positive lenses, and the 2-nd lens unit is composed of a pair of positive and negative lenses. In the embodiments of these zoom lens systems, many aspherical surfaces are used; most of the embodiments use at least one glass lens having aspherical surfaces on both sides thereof.
Prior art that uses plastic lenses includes Japanese Patent Application Laid-Open (KOKAI) No. 3-116110 (1991). Plastic lenses suffer from the problem that the focal point varies with changes in temperature-humidity conditions. Taking into consideration the problem of plastic lenses, the above-mentioned prior art combines together a pair of positive and negative plastic lenses so as to increase the composite focal length, thereby minimizing the variation of the focal point.
Under these circumstances, the present applicant has proposed a zoom lens system that uses a plastic lens in Japanese Patent Application No. 3-273985 (1991). In the proposed zoom lens system, the 1-st lens unit is composed of a lens of extremely small power, a negative lens and a positive lens.
Japanese Patent Application Laid-Open (KOKAI) No. 4-161914 (1992) discloses a zoom lens system that takes measures to minimize the variation of the focal point with changes in temperature-humidity conditions. In the disclosed lens arrangement, the 1-st lens unit is composed of a positive lens, a negative lens, a lens of small power, and a positive lens, and the zoom ratio is not higher than 2.
Prior zoom lens systems that have about six lens elements and use aspherical surfaces include Japanese Patent Application Laid-Open (KOKAI) Nos. 62-56917 (1987) and 64-42618 (1989). In these zoom lens systems, the 1-st lens unit is composed of a positive lens, a negative lens, a positive lens, and a positive lens, and the zoom ratio is of the order of two. In this case also, aspherical surfaces are used for glass lenses.
If it is intended to reduce the number of constituent lens elements in a zoom lens system for a compact camera, it is necessary to use a large number of aspherical surfaces and double-aspherical glass lenses as in the case of Japanese Patent Application Laid-Open (KOKAI) Nos. 3-127008 and 3-274516. Thus, these prior zoom lens systems are disadvantageous from the viewpoint of reducing the cost. Moreover, the requirement for accuracy is great because decentration of lens elements causes the performance to be degraded.
Even if it is intended to minimize the effect of variation in the focal point by combining a pair of positive and negative plastic lenses so that the composite focal length is increased, if the materials of these lenses are different from each other, the two lenses differ from each other in the tendency for the focal point to vary with changes in temperature-humidity conditions. If a change in configuration of each lens is also taken into consideration, it is extremely difficult to predict a variation of the focal point.
These problems have been solved by Japanese Patent Application No. 3-273985, proposed by the present applicant. In the proposed zoom lens system, however, the zoom ratio is limited to about two.
Even if a lens of small power is disposed in the 1-st lens unit as in the case of Japanese Patent Application Laid-Open (KOKAI) No. 4-161914, the basic arrangement of the 1-st lens unit is comprised of a positive lens, a negative and a positive lens. When the 1-st lens unit has such an arrangement, the power of the negative lens must be intensified in order to correct the Petzval's sum and spherical aberration. Moreover, distortion that is produced by the negative lens must be corrected with positive power. Consequently, the lens power becomes excessively strong, so that coma and astigmatism cannot satisfactorily be corrected. Therefore, the performance does not improve even if an aspherical lens of small power is disposed in the 1-st lens unit.
The above-described matters similarly apply to the arrangement of the 1-st lens unit in a zoom lens system composed of six lens elements and using aspherical surfaces as in the case of Japanese Patent Application Laid-Open (KOKAI) Nos. 62-56917 and 64-42618. That is, the power of the negative lens must be intensified in order to effect aberration correction. Particularly in Japanese Patent Application Laid-Open (KOKAI) No. 62-56917, since the power of the negative lens is excessively strong, the aspherical surface of the first positive lens must be arranged so that the positive power becomes stronger. Thus, since each lens has strong power in this type of zoom lens system, decentration of lens elements causes the performance to be degraded. Therefore, the requirement for assembling accuracy must be strict. In addition, aspherical surfaces are used for glass lenses, which is disadvantageous from the viewpoint of cost. Further, the zoom ratio is two, which is unsatisfactory.